Acetyl-coenzyme A carboxylases (ACCs) are crucial for the biosynthesis and oxidation of long-chain fatty acids, and they are important targets for the development of therapeutic agents against obesity, diabetes, and bacterial infections. See Alberts and Vagelos, 1972, The Enzymes, Boyer, ed., Academic Press, New York, vol. 6, pp. 37-82; Wakil et al., 1983, Ann. Rev. Biochem. 52:537-579; McGarry and Brown, 1997, Eur. J. Biochem. 244:1-14; Abu-Elheiga et al., 2001, Science 291:2613-2616; Ramsay et al., 2001, Biochim. Biophys. Acta 1546:21-43; Cronan, Jr. and Waldrop, 2002, Prog. Lipid Res. 41:407-435; Lenhard and Gottschalk, 2002, Advanced Drug Delivery Reviews 54:1199-1212. In addition, the carboxyltransferase (CT) domain of this enzyme from some plants is the site of action of widely-used commercial herbicides such as haloxyfop and sethoxydim (Gronwald, 1991, Weed Science 39:435-449; Devine and Shukla, 2000, Crop Protection 19:881-889; Zagnitko et al., 2001, Proc. Natl. Acad. Sci. USA 98:6617-6622; Delye et al., 2003, Plant Physiol. 132:1716-1723).
ACCs catalyze the formation of malonyl-CoA from acetyl-CoA and CO2, a reaction that also requires the hydrolysis of ATP. Two isoforms of this enzyme have been identified in mammals. ACC 1, a cytosolic enzyme, catalyzes the committed step in the biosynthesis of long-chain fatty acids. Wakil et al, 1983, Ann. Rev. Biochem. 52:537-579. In comparison, ACC2 is associated with the mitochondrial membrane and its malonyl-CoA product potently inhibits the shuttle that transports long-chain acyl-CoAs from the cytosol to the mitochondria for oxidation. McGarry and Brown, 1997, Eur. J. Biochem. 244:1-14; Ramsay et al., 2001, Biochim. Biophys. Acta 1546:21-43.
The malonyl-CoA produced in the ACC-catalyzed reaction is a negative regulator of carnitine palmitoyltransferase 1, which is involved in fatty acid oxidation. During starvation, ACC levels and consequently malonyl-CoA levels are decreased and fatty acid oxidation is increased. The reduction of malonyl-CoA results in an increase in ATP synthesis which is directly linked to an increase in fatty acid oxidation, and also results in a decrease in ATP consumption for fatty acid synthesis which is consequently decreased. Interestingly, mice lacking ACC2 exhibit a higher rate of fatty acid oxidation and reduced body fat and body weight, while genetic ablation of ACC1 in mice was found to be embryonically lethal, possibly due to lack of C2 units for the synthesis of fatty acid needed for biomembrane synthesis. See Abu-Elheiga et al., Science 291:2613-2616 (2001); see also U.S. Patent Application Publication No. 20030028912 of Matzuk et al. published Feb. 6, 2003.
Mammalian, yeast, and most other eukaryotic ACCs are large, multi-functional enzymes, containing the biotin carboxylase (BC) domain, the biotin carboxyl carrier protein (BCCP) domain, and the carboxyltransferase (CT) domain. See FIG. 1A. BC catalyzes the ATP-dependent carboxylation of a biotin group covalently linked to a lysine residue in BCCP, and then CT catalyzes the transfer of the carboxyl group from biotin to acetyl-CoA to produce malonyl-CoA. In E. coli and other bacteria, ACCs are multi-subunit enzymes composed of 3 distinct protein subunits, with a BC subunit, a BCCP subunit, and two subunits for the CT. See FIG. 1A. Crystal structures are available for the BC and BCCP subunits of E. coli ACC (see Cronan Jr. and Waldrop, 2002, Prog. Lipid Res. 41:407-435).